Solid state drives (SSDs) are a type of data storage device that uses integrated circuit assemblies to store data persistently. The term “solid state” refers to the fact that SSDs have no moving mechanical components, unlike traditional hard disk drives (HDDs) which use spinning platters and read/write heads.
What is a solid state drive?
A solid state drive is a data storage device that uses flash memory to store data digitally. Flash memory is a type of electronically erasable programmable read-only memory (EEPROM) that can be electrically erased and reprogrammed. It is non-volatile, meaning it retains stored data even when powered off.
SSDs use interconnected flash memory chips that retain data in the absence of power. This flash memory is arranged in cell blocks that can be electronically erased and rewritten. Because of this, SSDs have no moving parts unlike traditional electromechanical magnetic disk drives.
How do SSDs work?
SSDs use a controller that interfaces between the flash memory components and the host computer. The controller performs several core functions:
- Accepts read and write commands from the host
- Manages the flash memory including reading, writing, erasing, and refreshing blocks
- Handles error correction by fixing bad bits
- Handles wear leveling to distribute writes across the cells evenly
- Buffers data for transferring to and from the flash memory
- Interfaces with the host computer via SATA, PCIe or other connections
When data is written to the SSD, it is broken down into pages then programmed to multiple cells in parallel. When data is read, the controller locates the desired data in the flash memory and transfers it via the interface protocol.
Advantages of SSDs
SSDs provide several key advantages over traditional hard disk drives (HDDs):
- Faster read/write speeds – SSDs have much faster I/O performance compared to HDDs because they can access data electronically instead of mechanically.
- Higher reliability – With no moving parts, SSDs have lower failure rates compared to HDDs with spinning disks and heads.
- Lower latency – SSDs can access data almost instantly, while HDDs require time for the head to move to the right track and sector.
- Lower power consumption – SSDs consume less power than HDDs which require electricity to spin up the platter.
- Lighter weight – SSDs weigh much less than HDDs with heavy magnetic platters.
- Compact size – 2.5″ SSDs provide more storage capacity in a smaller footprint compared to HDDs.
- Resistant to shock/vibration – Without sensitive moving parts, SSDs can withstand more shocks.
Disadvantages of SSDs
While SSDs provide significant advantages, they also have some limitations compared to HDDs:
- Higher cost per gigabyte – SSDs are more expensive than HDDs in terms of cost per gigabyte.
- Lower capacities – HDDs are available in larger capacities up to tens of terabytes, while SSD capacities top out at around 100TB.
- Wear out over time – SSD cells can wear out after a certain number of erases, while HDDs retain data indefinitely.
- File fragmentation – SSD performance can degrade with long term file fragmentation, unlike defragmented HDDs.
SSD Form Factors
SSDs are commonly available in several form factors, interfaces, and capacities catered to different applications:
| Form Factor | Size | Interface | Capacity | Usage |
|---|---|---|---|---|
| 2.5″ SSD | 69.85 x 100 mm | SATA | Up to 8TB | Laptops, desktops |
| M.2 SSD | 22 x 30/42/60/80 mm | PCIe, SATA | Up to 4TB | Ultrabooks, tablets |
| mSATA SSD | 50.8 x 29.85 mm | SATA | Up to 1TB | Compact devices |
| U.2 SSD | 15 x 70 mm | PCIe | Up to 15TB | Data centers |
The most common SSD form factors are 2.5″ and M.2. 2.5″ drives use the standard SATA interfaces and can reach maximum capacities around 8TB. M.2 SSDs use PCIe and SATA interfaces, with smaller dimensions optimal for space-constrained devices.
Flash Memory in SSDs
The flash memory chips that make up the data storage arrays in SSDs come in several different varieties:
- SLC (single-level cell) – Stores 1 bit per cell, fastest performance
- MLC (multi-level cell) – Stores 2 bits per cell, mainstream SSDs
- TLC (triple-level cell) – Stores 3 bits per cell, slower but higher capacities
- QLC (quad-level cell) – Stores 4 bits per cell, slowest but highest capacities
Higher bit density chips like TLC and QLC offer greater storage density and cheaper cost per gigabyte, but have slower write speeds. SLC and MLC provide faster performance, but lower capacities and higher cost.
NAND vs NOR Flash
Along with cell levels, flash memory in SSDs use different architectures:
- NAND flash – Higher density, lower cost, written in blocks. Used for data storage.
- NOR flash – Lower density, higher cost, written byte-wise. Used for code storage.
SSDs predominantly use NAND flash optimized for high capacity data storage. NOR flash offers random access reads needed for executing code, but lower storage density and higher cost.
Interface Protocols
SSDs use several standard device interfaces to connect to host computers and transfer data:
- SATA – Compatible with HDD interface, limited to 6Gbps speeds.
- PCIe – Direct connection to PCIe bus, performance up to 16GBps.
- SAS – Enterprise version of SATA, 12Gbps speeds.
- NVMe – Optimized protocol for PCIe SSDs, lower latency.
SATA has been the most common SSD interface, but PCIe and NVMe are growing in popularity for their faster bandwidth. SAS offers higher performance than SATA for enterprise use.
3D NAND and V-NAND Technology
To increase densities further, SSD manufacturers have developed 3D NAND flash technology, stacking memory cells vertically:
- 3D NAND – Flash cells stacked vertically in 32+ layers.
- V-NAND – Samsung’s brand of 3D NAND flash memory.
This 3D vertical stacking provides greater storage density in a smaller footprint compared to planar NAND flash. V-NAND is Samsung’s pioneering 3D flash technology that stacks cells up to 128 layers high.
DRAMless and Host Memory Buffer SSDs
Many SSDs integrate a DRAM chip to cache mappings between logical block addresses and physical NAND locations. Newer SSDs are eliminating this DRAM to lower costs:
- DRAMless SSDs – Use host memory for flash translation layer mapping, rather than onboard DRAM.
- Host Memory Buffer SSDs – Leverage host DRAM as an extended cache/buffer for the SSD controller.
By relying on the host computer’s DRAM instead of onboard DRAM chips, SSDs can reduce their bill of materials costs. However, DRAMless designs can result in slower random write speeds.
SSD Reliability and Endurance
SSDs can only erase and re-write flash memory cells a finite number of times before they wear out and become unreliable. To account for this, SSDs include:
- Wear leveling – Distributes writes across cells evenly to avoid early failures.
- Bad block management – Maps out failed/worn out cells to avoid using them.
- ECC – Error correcting codes to fix a certain number of bit errors.
- Over-provisioning – Extra capacity is hidden to replace worn out cells.
In addition, SSD life spans are measured by drive writes per day (DW/D) or total terabytes written (TBW) over the warranty period. Typical consumer SSDs offer 5 years with TBW ratings from 150TB to 600TB for higher end models.
SSD Performance Factors
Several factors affect overall SSD speeds and benchmark performance:
- Interface – SATA, PCIe, and NVMe offer wide speed ranges.
- Controller – Quality and number of channels/cores impact speeds.
- NAND Type – SLC is fastest. TLC and QLC are slower but more affordable.
- Capacity – Higher capacity SSDs often have faster write speeds.
- Over-provisioning – More spare area improves write speeds.
- Workloads – High sequential reads/writes show best performance.
Understanding how these SSD architecture choices affect real-world speeds helps match the right SSD type for specific workloads and use cases.
SSD Caching – Hybrid Drives
Some solid state hybrid drives use a small SSD as a cache inside a traditional HDD:
- SSHD (solid state hybrid drive) – Combines HDD capacity with SSD caching of hot data.
- Seagate FireCuda – Seagate brand of SSHD with 8GB to 128GB of NAND flash.
The SSD component acts as a cache to store frequently accessed “hot” data to improve real-world performance over HDDs alone. However, the overall benefit relies heavily on the predictive cache algorithms.
Differences Between SSDs and HDDs
While both SSDs and HDDs provide long-term data storage, there are key differences between solid state drives and mechanical hard disk drives:
| SSD | HDD | |
|---|---|---|
| Storage medium | NAND flash memory | Magnetic platters |
| Latency | Microseconds | Milliseconds |
| Interface | SATA, PCIe, NVMe | SATA, SAS |
| Read/write | Faster | Slower |
| Shock resilience | Much higher | Low |
| Noise | Silent | Spinning platters |
| Price per GB | Higher | Lower |
In summary, SSDs provide faster access, higher resilience, lower power draw, and silent operation but at a higher price point. HDDs offer larger capacities at lower costs but slower performance.
Conclusion
Solid state drives provide fast, reliable data storage and retrieval through the use of flash memory chips and integrated circuit controllers. Their solid state design with no moving parts gives SSDs key advantages over traditional electromechanical hard disk drives.
Factors like NAND flash type, controller, and interface all impact SSD performance. 3D NAND stacking has further increased the storage density available in high capacity SSDs. For most consumer and business use cases, SSDs now offer a compelling advantage over HDDs in terms of speed, durability, form factor, power efficiency, and noise.
Trends like shrinking process nodes, QLC NAND, Host Memory Buffer, and PCIe/NVMe are rapidly evolving SSD technology and performance. SSDs have become the standard for client storage. While HDDs still have a role for high capacity bulk storage, solid state drives are now dominating many storage applications.